c++: ttp matching with constrained auto parm [PR99909]

c++: ttp matching with constrained auto parm [PR99909]

Here, when unifying TT with S, processing_template_decl is unset, and
this foils the dependence checks in do_auto_deduction for avoiding
checking constraints on an auto when the initializer is dependent.

This patch fixes this by making sure processing_template_decl is set
around the call to unify from coerce_template_template_parms; this seems
sensible because we're unifying one set of template parameters with
another, so we're dealing with templated trees throughout.

	PR c++/99909

gcc/cp/ChangeLog:

	* pt.c (coerce_template_template_parms): Keep
	processing_template_decl set around the call to unify as well.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-ttp3.C: New test.